The present invention relates to a power steering fluid for a fluid power steering gear for a vehicle, and particularly to a power steering fluid additive for use in a power steering fluid.
A fluid power rack and pinion vehicle steering gear commonly includes a rack that extends axially through a chamber. The rack is preferably made from a metal, such as steel. The ends of the rack project axially outward from the ends of the chamber. Steering linkage is connected to the projecting ends of the rack and to the steerable wheels of the vehicle.
A piston is fixed to the rack within the chamber. Rotation of the vehicle steering wheel actuates a valve that causes power steering fluid under pressure to act against the piston. The force exerted by the fluid moves the piston within the chamber and moves the rack axially. Axial movement of the rack moves the steering linkage to turn the steerable wheels of the vehicle. The ends of the chamber through which the rack projects are sealed with suitable seals to prevent fluid leakage from the chamber.
Axial movement of the metal rack creates friction between the seals and the metal rack. Friction between the seals and the metal rack causes the surfaces of the seals to stick and slip against the surface of the metal rack. The sticking and slipping of the seals cause the seals to vibrate at a frequency of about 100 to about 200 Hertz, which results in noise in the human hearing range.
Commercially available power steering fluids, when used in a power steering gear, provide lubrication between the seals and the metal rack. The amount of lubrication, however, is insufficient to eliminate noise generated by axial movement of the metal rack.
Commercially available power steering fluids include mineral oil similar to kerosene. Mineral oil is a poor lubricant and has a high viscosity at low temperatures. The pour point of a mineral oil is typically in the range of xe2x88x9225xc2x0 C. Below about xe2x88x9225xc2x0 C., mineral oil is semisolid and not useful to transmit hydraulic power. Wax is an excellent lubricant for rubber/steel interfaces and can be added to mineral oil. Wax, however, when added to mineral oil, increases the viscosity of the mineral oil making the mineral oil unsuitable for use as a power steering fluid.
The present invention is a fluid power steering gear. The fluid power steering gear comprises a housing. A power steering fluid is disposed within the housing. A seal contains the power steering fluid within the housing. A member extends through the housing and the seal. The member is movable relative to the housing and the seal in response to a change in the fluid pressure in the housing. The power steering fluid comprises a base oil and a metal-free lubricant additive. The metal-free lubricant additive is soluble in the base oil and modifies the interfacial surface tension between the base oil and the member and the base oil and the seal. The weight percent of the metal-free lubricant additive is about 0.1% to about 5.0%, by weight of the power steering fluid.
In accordance with one embodiment of the present invention the metal-free lubricant additive comprises a fatty acid salt of a secondary amine.